Polytetrafluoroethylene porous membrane with small elongation anisotropy and process for production thereof

Abstract

The present invention provides a polytetrafluoroethylene (PTFE) porous membrane having, when an arbitrary in-plane direction of the membrane is taken as a first direction, and an in-plane direction orthogonal to the first direction is taken as a second direction, a strength of 20 N/mm 2 or more, an elongation percentage at breakage of 200% or less, and a ratio of the elongation percentage in the second direction with respect to the elongation percentage in the first direction of 0.5 to 2.0, in a tensile test performed in the first direction and in the second direction. The PTFE porous membrane of the present invention is less elongated due to an external force and has small anisotropy in elongation.

1 . A polytetrafluoroethylene porous membrane having, when an arbitrary in-plane direction of the membrane is taken as a first direction and an in-plane direction orthogonal to the first direction is taken as a second direction:
 a strength of 20 N/mm 2  or more,   an elongation percentage at breakage of 200% or less, and   a ratio of the elongation percentage in the second direction with respect to the elongation percentage in the first direction of 0.5 to 2.0, in a tensile test conducted in the first direction and the second direction, wherein   the tensile test is performed, using a sample with a length in a measurement direction set to 5 cm and a length in a direction perpendicular to the measurement direction set to 1 cm, while the sample is supported by a pair of chucks with an initial distance between the chucks maintained at 2 cm, by separating the pair of chucks from each other at a speed of 20 cm/minute.   
     
     
         2 . The polytetrafluoroethylene porous membrane according to  claim 1 , wherein
 a heat absorption peak associated with crystal melting is present only in a range of 327±5° C., and an absorption enthalpy of the heat absorption peak is less than 35 J/g, in differential scanning calorimetry at a temperature rise rate of 10° C./minute.   
     
     
         3 . The polytetrafluoroethylene porous membrane according to  claim 1 , having a thickness of 5 μm or more and a porosity of 50% or more. 
     
     
         4 . The polytetrafluoroethylene porous membrane according to  claim 1 , having a permeation of 0.01 to 200 sec/100 mL, in terms of Gurley number. 
     
     
         5 . A process for producing a polytetrafluoroethylene porous membrane, comprising the step of:
 pressurizing a stack including at least two polytetrafluoroethylene porous membranes that have been made porous by biaxial stretching in a state where an in-plane direction of one of the membranes in which an elongation percentage at breakage in a tensile test is minimum is substantially orthogonal to an in-plane direction of the other of the membranes in which an elongation percentage at breakage in the tensile test is minimum, under heating to a temperature equal to or higher than a crystal melting temperature of polytetrafluoroethylene so as to be integrated, wherein   the tensile test is performed, using a sample with a length in a measurement direction set to 5 cm and a length in a direction perpendicular to the measurement direction set to 1 cm, while the sample is supported by a pair of chucks with an initial distance between the chucks maintained at 2 cm, by separating the pair of chucks from each other at a speed of 20 cm/minute.   
     
     
         6 . The process for producing a polytetrafluoroethylene porous membrane according to  claim 5 , wherein
 the at least two polytetrafluoroethylene porous membranes that have been made porous by biaxial stretching have a crystal melting enthalpy of at least 35 J/g and not more than 70 J/g, as measured by differential scanning calorimetry at a temperature rise rate of 10° C./minute.